When sealing envelopes, moisture must be applied to the gummed regions of their flaps in a manner which is quick and reliable, and independent of the types and formats of the envelopes. Advantageously, this moistening operation should be performed while the envelopes are being transferred from the outlet of an envelope un-stacking device which serves to feed envelopes one by one with their flaps open to the inlet of a franking machine. In addition to operating reliably and rapidly, such moistening devices must also leave sufficient moisture on the envelope flaps to ensure that they are properly sealed while avoiding the application of too much moisture in order to avoid drops and smudges forming while the envelopes are being sealed.
Several prior devices for moistening envelope flaps are already known. They comprise a tank of moistening liquid (generally water) and a moistening member having a first portion which extends some way into the liquid tank and a second portion which "licks" the flap of each envelope. The envelope flaps are thus moistened as they are driven in a flat position over the moistening device on a displacement mechanism or plane having an edge beyond which the envelope flaps are caused to hang freely. In order to ensure that liquid is effectively transferred to the flaps, they are pressed against the moistening member by means of a spring-biased deflector plate or the like disposed opposite the moistening member.
One such device for moistening envelope flaps is described in French published patent application No. 2 332 813. In this device, the moistening member is constituted by an endless belt which is driven to transfer the moistening liquid from the liquid tank to the envelope flaps as the flaps pass close to the belt and are pressed thereagainst over the full lengths of their gummed portions.
Another such a moistening device is described in published French patent application No. 2 195 532, in which the moistening member constituted by a wick held on a wick support arm and having one of its ends immersed in the liquid in the tank and having its other end in the form of an elongate wetting portion standing out from the support arm. In this embodiment moisture is transferred along the wick by capillarity in the wick-constituting material, and the flap is moistened by rubbing against the end wetting portion of the wick. It is difficult to regulate the transfer of moisture from the end wetting portion of the wick to the gummed flaps of the envelopes in this kind of device. The transfer depends on the force with which the flaps are pressed against the wetting portion of the wick, and this force is itself a function of the thickness of the envelope flaps, which may mean that the flap of a thin envelope is insufficiently moistened while the flap of a thick envelope is excessively moistened, and which may also give rise to the envelopes being damaged by virtue of their flaps being slowed down.
Such friction also causes gum to be transferred from the flaps to the wetting portion of the wick, and in the course of time this can give rise to uneven transfer of moisture to envelope flaps. The transfer of moisture to envelope flaps also depends on the transfer of moisture along the wick itself, which transfer is obtained by capillarity in the wickconstituting material, and is therefore sensitive to the use of hard water as the moistening liquid (which is quite common). Furthermore, in this prior embodiment the amount of moisture transferred to the flaps varies along the length of the flaps, giving rise to problems with long envelope flaps, whose leading ends may be excessively moistened, and whose trailing ends may be insufficiently moistened.
Preferred implementations of the present invention seek to provide a moistening device which avoids the above-mentioned drawbacks by providing uniform moistening of envelope flaps regardless of their speed of displacement and of their lengths, furthermore this is achieved with practically no risk of damaging the envelopes and without the amount of moistening liquid transferred changing appreciably over time.